Ferromagnetic-antiferromagnetic coexisting ground states and exchange bias effects in \(\bf{MnBi_4Te_7}\) and \(\bf{MnBi_6Te_{10}}\)

Natural superlattice structures $\rm{(MnBi_2Te_4)(Bi_2Te_3)}$$_n\( (\)n\( = 1, 2,...), in which magnetic \)\rm{MnBi_2Te_4}\( layers are separated by nonmagnetic \)\rm{Bi_2Te_3}\( layers, hold band topology, magnetism and reduced interlayer coupling, providing a promising platform for the realization of exotic topological quantum states. However, their magnetism in the two-dimensional limit, which is crucial for further exploration of quantum phenomena, remains elusive. Here, complex ferromagnetic (FM)-antiferromagnetic (AFM) coexisting ground states that persist up to the 2-septuple layers (SLs) limit are observed and comprehensively investigated in \)\rm{MnBi_4Te_7}\( (\)n\( = 1) and \)\rm{MnBi_6Te_{10}}\( (\)n\( = 2). The ubiquitous Mn-Bi site mixing modifies or even changes the sign of the subtle inter-SL magnetic interactions, yielding a spatially inhomogeneous interlayer coupling. Further, a tunable exchange bias effect is observed in \)\rm{(MnBi_2Te_4)(Bi_2Te_3)}$$_n\( (\)n\( = 1, 2), arising from the coupling between the FM and AFM components in the ground state. Our work highlights a new approach toward the fine-tuning of magnetism and paves the way for further study of quantum phenomena in \)\rm{(MnBi_2Te_4)(Bi_2Te_3)}$$_n\( (\)n$ = 1, 2,...) as well as their magnetic applications.